Downgrade -Werror=undef to a warning -Wundef (#711)

Author: Bodigrim

bytestring.cabal

@@ -150,7 +150,7 @@ library
                    Data.ByteString.Utils.UnalignedAccess
 
   ghc-options:     -Wall -fwarn-tabs -Wincomplete-uni-patterns
-                   -optP-Wall -optP-Werror=undef
+                   -optP-Wall -optP-Wundef
                    -O2
                    -fmax-simplifier-iterations=10
                    -fdicts-cheap
@@ -178,7 +178,7 @@ library
     -- DNDEBUG disables asserts in cbits/
     cc-options:        -std=c11 -DNDEBUG=1
                        -fno-strict-aliasing
-                       -Werror=undef
+                       -Wundef
 
     -- No need to link to libgcc on ghc-9.4 and later which uses a clang-based
     -- toolchain.